With continual increase in the number of handheld devices, such as cellular phones and media players, that are currently in use, there is a need for compact input devices to navigate through different options available on these devices. A popular input device on handheld devices is a click wheel, which is based on capacitive sensing. The click wheel includes a ring-shaped membrane with conductive channel grid (conductors) on the top surface and multiple conductive wedges on the bottom surface. The multiple conductive wedges are electrically connected to a capacitive sensing integrated circuit (IC).
In operation, current is supplied to the conductive channel grid on the top surface of the click wheel membrane. When a finger (a conductor and a path to ground) is placed on the click wheel near the conductive channel grid, current flows through the finger from the conductive channel grid near the finger, which is sensed by the capacitive sensing IC via the conductive wedges on the bottom surface. As the finger moves around the click wheel, the capacitive sensing IC detects the corresponding capacitive change, which uses this information to track the movement of the finger.
A concern with the click wheel is that each of the conductive wedges requires a pin on the capacitive sensing IC. As an example, if the click wheel has sixteen conductive wedges, the capacitive sensing IC needs to have sixteen pins to provide sixteen counts per rotation. As IC fabrication process geometries improve and logic die size decreases, realization of the associated cost reduction for the capacitive sensing IC may not be possible because of the pin count requirement for the capacitive sensing IC. Thus, the high pin count may prove to be a barrier to cost reduction.
Thus, there is a need for a lower cost input device, which can provide similar functionality of a click wheel, for use in handheld devices.